Note [Weird special case for SpecDict]

Suppose we are trying to specialise for this this call:
   $wsplit @T (mkD @k @(a::k) :: C T)
where
   mkD :: forall k (a::k). C T
is a top-level dictionary-former.  This actually happened in #22459,
because of (MP1) of Note [Specialising polymorphic dictionaries].

How can we specialise $wsplit?  We might try

   RULE "SPEC" forall (d :: C T). $wsplit @T d = $s$wsplit

but then in the body of $s$wsplit what will we use for the dictionary
evidence?  We can't use (mkD @k @(a::k)) because k and a aren't in scope.
We could zap `k` to (Any @Type) and `a` to (Any @(Any @Type)), but that
is a lot of hard work for a very strange case.

So we simply refrain from specialising in this case; hence the guard
   allVarSet (`elemInScopeSet` in_scope) (exprFreeVars d)
in the SpecDict cased of specHeader.

How did this strange polymorphic mkD arise in the first place?
From GHC.Core.Opt.Utils.abstractFloats, which was abstracting
over too many type variables. But that too is now fixed;
see Note [Which type variables to abstract over] in that module.